This invention relates to a method for protecting a vessel's loading space from excess pressure. More particularly it concerns a method for protecting a loading space provided with cargo pressure tanks from excess pressure if a leak should occur in a cargo pressure tank or piping belonging thereto. The invention also includes a device for practicing the method. By loading space is meant in this context the ship's hold and adjacent rooms, for example a valve compartment and a piping room.
The shipping of gaseous petroleum products has essentially been carried out by means of the so-called “Liquified Natural Gas Method” (Liquified Natural Gas—LNG). The method comprises the cooling of gas to a liquid form, after which the gas can be transported in ship tanks at atmospheric pressure. Expensive equipment is required at both the point of shipment and the terminal for the treatment of the gas. Since the gas has to be cooled to a relatively low temperature, up to one fifth of the gas is spent on running the cooling and heating processes. Energy consumption like that just for the transport-related processes is expensive and moreover environmentally alarming.
Several other ship-based solutions have been proposed, wherein the gas is pressurized and/or cooled in order to achieve a gas density practical for the purpose. Such solutions have not become widely used in practice, but a solution in which a large number of vertical, tubular pressure tanks are placed in a ship's hold, has drawn considerable attention. The method is referred to as “Pressurised Natural Gas” (PNG). In accordance with such a method the gas is compressed at the point of shipment to an overpressure of a few hundred bars, and the gas is then filled into the cargo pressure tanks located on the ship. The cooling is limited to a simple and cheap removal of the compression heat from the gas, so that the transport temperature will be close to the ambient temperature.
By the use of cargo pressure tanks and associated piping, which are subjected to a relatively high pressure during the transport, it is of great importance in terms of safety that gas that might flow out of the cargo pressure tank or the piping by a possible leak, can be dealt with safely, without this involving the risk of damage to the rest of the cargo pressure tanks or the ship.
In the planning of a ship's configuration, possible unintended events that might occur must be analysed, after which the ship is designed with technical solutions arranged to relieve such events.